An aircraft lifting surface usually comprises a torsion box as its main supporting structure. For example, an aircraft tail plane (horizontal or vertical) is usually structured by a leading edge, a torsion box and a trailing edge with control surfaces (flaps, elevators, rudders, etc.). The torsion box is the main supporting structure responsible for supporting all loads involved (aerodynamic, fuel, dynamics, etc.) and comprises several structural elements.
Composite materials with an organic matrix and continuous fibers, especially CFRP (Carbon Fiber Reinforced Plastic), are nowadays widely used in the aeronautical industry in a great variety of structural elements. Specifically, all the elements which make up the torsion boxes of aircraft tail planes can be manufactured using CFRP.
The design of composite torsion boxes requires combining two perspectives of different nature: that of structural design and that of manufacture.
The traditional approach is the design of the torsion box defining the structural elements that form it (skins, spars, stringers, ribs), the separate manufacture of these elements and their subsequent joint in the assembly plant following schemes similar to those used in the aeronautical industry when only metallic materials were used.
The manufacture can be done using prepreg technology. In a first step, a flat lay-up of composite prepreg plies for each element is prepared. Then a laminated preform of the element with the required shape is obtained by means of a classical hot-forming process, being in some cases substituted by a press-forming process due to high curvatures. After getting the required shape, the laminated preform is cured in a male or female tooling depending on the tolerances required and the overall manufacturing cost. In the case of certain elements comprising sub-components cured separately, such as a rib and a vertical stiffener of it, a second curing cycle is needed for co-bonding said sub-components. Finally, after all the curing cycles, the element contours are trimmed getting the final geometry, and then the element is inspected by an ultrasonic system to assure its quality. The cost of a torsion box manufactured with said method is high because said steps shall be carried out independently for each structural element. Additionally, the cost related to the assembly of the torsion box is also high due to the long length and high complexity of the tasks required to install and to fit all structural elements together. This approach is being followed for manufacturing multi-rib torsion boxes such as that of the horizontal tail plane (HTP) shown in FIGS. 1a, 1b, 1c. 
The HTP is structured by leading edges 11, torsion boxes 13 and trailing edges 15 with control surfaces (flaps, elevators, rudders, etc.).
The leading edge is the structure responsible for keeping the aerodynamic surface with the torsion box surface, for supporting the static or cyclic structural loads involved and for protecting the torsion box from bird impacts. It is the part of the HTP surface that first contacts the air and the foremost edge of the airfoil.
A known leading edge 11 comprises, on the one side, several ribs 10, called leading edge ribs, attached to the front spar 18 of the torsion box 13 and, on the other side, an aerodynamic profile 12—commonly known as “nose”—attached to the leading edge ribs 10 and to the flanges of the front spar 18 in order to keep the overall aerodynamic shape of the HTP.
Similarly the trailing edge 15 comprises, on the one side, several ribs, called trailing edge ribs attached to the rear spar 20 of the torsion box 13 and, on the other side, an aerodynamic profile 16 attached to the trailing edge ribs and to the flanges of the rear spar 20 in order to keep the overall aerodynamic shape of the HTP between the torsion box and the control surfaces.
The structural elements of torsion boxes 13 are upper and lower skins 21, 23 stiffened by longitudinal stringers, a front spar 18, a rear spar 20 and transverse ribs 17 attached to the front and rear spars 18, 20 and to the upper and lower skins 21, 23 in order to keep the torsion box shape and reinforce the load introductions areas linked to the HTP structural arrangement in the aircraft and to the actuators for handling the HTP control surfaces.
An alternative approach is to manufacture the whole or a part of a torsion box in an integrated manner for obtaining a monolithic ensemble comprising all or part of the structural elements of the torsion box. In this respect one example is described in WO 2008/132251 for a multi-spar torsion box.
Due to the complexity of aircraft tail planes the aeronautics industry is constantly demanding new proposals and new manufacturing methods that improve efficiency and/or costs of known aircraft tail planes.
The present invention is directed to the attention of that demand.